Adult and pediatric patients who are maintained by a regime of positive pressure ventilation are likely to develop various complicating conditions related to the use of this type of ventilation. These include barotrauma of various kinds such as pneumothorax, pneumomediastinum, pneumopericardium, pneumoperitoneum, subcutaneous emphysema and air embolization.
The act of intubation itself presents hazards in ventilation including disconnection, inadvertent extubation, tracheal trauma, infection, tube blockage, vocal cord dysfunction and subglottic stenosis. Intubation also is a highly skilled procedure.
Negative pressure ventilators of the "iron lung" and "iron cuirass" type) for adult patients and children have long been available and were the first type of ventilators to be developed. These functioned well for patients such as those affected by poliomyelitis and other neuro-muscular disorders where the lungs are essentially healthy but their function is disrupted by impairment of neurologic function, muscle contraction etc. They were much less successful in conditions such as adult respiratory distress syndrome (ARDS) where the lungs themselves contain the primary defects including reductions in pulmonary compliance and increased airway resistance. They have accordingly very largely fallen out of use in favour of a variety of positive pressure ventilation regimes despite the problems attendant upon the use of positive pressure.
Generally, such negative pressure ventilators operate at a low frequency such as 10 to 20 breathing cycles per minute. In the negative pressure cuirass type ventilator described in U.S. Pat. No. 3,078,842, a pressure alternator provides pressure variations at a frequency of 10 to 20 per minute to produce ventilation, whilst a second pressure alternator superimposes periodically a very high pressure at a higher frequency (60 to 120 pulses per minute) to produce cardiac massage. This device is intended for resuscitation from pulmonary and cardiac arrest and not for prolonged ventilation. The high frequency, high pressure aspect of the treatment is to stimulate the heart and is not suitable in itself to produce ventilation.
Particular difficulties arise in the ventilation of neonate and preterm babies.
Neonate and preterm babies with respiratory failure develop hypoxia, and metabolic and respiratory acidosis that may lead to their death if untreated by assistant ventilation.
At present, neonates needing to be ventilated are generally intubated and respiration is then forced by positive pressure applied to the lung through the intubation. This procedure carries with it a serious risk of barotrauma as described above, in particular, pneumothorax, pneumomediastinum, interstitial emphysema, or bronchopulmonary displasia (BPD). A very high proportion of babies ventilated by this method, known as intermittent positive pressure ventilation (IPPV), will develop BPD caused directly by this procedure. There is also a danger of causing laryngial and tracheal complications and of introducing infection into the lung.
As many as fifteen percent of babies ventilated by IPPV have the complication of interstitial emphysema which carries a high mortality rate.
The IPPV procedure has however been found preferable in many circumstances to the use of ventilators of the kind described for instance in U.S. Pat. No. 2863447 which is a negative pressure ventilator having an incubator forming a pressure chamber divided into two compartments by a flexible seal. The head of the infant is contained in one compartment and the body in the other and the seal makes a substantially gas tight seal about the infant's neck. It is then possible to produce a cyclic variation of pressure in the body compartment and optionally also the head compartment. Partial evacuation of the body compartment with or without simultaneous increase of pressure in the head compartment causes expansion of the lungs and release of air into the body compartment allows the lungs to expel air. Generally, such negative pressure ventilators operate at a rate of from 20 to 60 cycles per minute.
Such negative pressure ventilators have been found to have several severe drawbacks some of which relate to the structure of the incubator.
First, it is almost impossible to use such ventilators to ventilate babies having a weight of less than 1.5 kilograms. The pressure differential between the atmosphere surrounding the head and the interior of the body compartment produces forces trying to suck the baby's head through the flexible seal and this imposes an excessive strain on the baby's neck in the case of very small babies.
Secondly, the patient is inaccessible for routine or emergency procedures. Being entirely contained within the ventilator which must be kept closed if ventilation is to continue, the patient is wholly inaccessible. For installing or maintaining drips or arterial lines and even for simple operations such as cleaning and nappy changing, the ventilator must be opened and the patient must be intubated.
Thirdly, the operation of the ventilator produces a constant flow of air into and out of the body cavity of the ventilator which produces a cooling effect which is difficult to counteract. Very small babies are of course very prone to suffer severe heat loss.
Negative pressure ventilators of this kind proposed in the past are relatively expensive because they involve an entire incubator.
Most seriously however, negative pressure ventilators of previously known designs do not provide maintain clinical parameters at acceptable values in actual use in treating patients with lung disorders and they have not found clinical acceptance. Despite the known problems, IPPV techniques are still the mainstay of clinical practice in this field.
U.S. Pat. No. 3,903,869, (Bancalari) discloses a continuous negative pressure chamber for treating infants with ideopathic respiratory distress syndrome (IRDS). The chamber receives the trunk of an infant, seals being provided at the neck and abdomen.
The chamber of U.S. Pat. No. 3,903,869 is not in fact intended to produce forced respiration but rather to assist spontaneous breathing. In some embodiments, it provides a constant negative pressure to prevent lung collapse. In the embodiment described with reference to FIG. 4, provision is made for increasing negative pressure cyclically at up to 30 to 40 cycles per minute to induce spontaneous breathing by stirring the infant out of apnea.
Whilst both positive pressure and negative pressure ventilation have traditionally operated at frequencies similar to those of natural breathing, more recently techniques of high frequency positive pressure ventilation (HFPPV) have been proposed, although not widely accepted. In such methods, ventilation is conducted at above 1 Hz. It was hoped that the small tidal volumes and generally lower airway pressures developed by high frequency ventilators would be associated with a lower incidence of complications, but experience has not borne this out. Interest in this technique was widespread for a brief period but is now decreased.
Little is known about the mechanisms by which oxygenation and ventilation occur during HFPPV although a number of plausible theories have been proposed.
Some experimental work on healthy animals and healthy animal lung tissue has been conducted using brief periods of external high frequency ventilation, but until now there has been no demonstration of a technique of this type capable of providing satisfactory ventilation for prolonged periods of healthy lungs nor of a sick lung.
Ward et al (J. Appl. Physiol: Respirat. Environ. Exercise Physiol. 54 (2): 427-433, 1983) applied external high frequency oscillatory ventilation to isolated, perfused rat lung and concluded that satisfactory oxygen uptake could be maintained by this method.
Hart et al (J. Appl. Physiol: Respirat. Environ. Exercise Physiol. 56 (1): 155-160, 1984) compared external and internal (tracheal) high frequency ventilation for five minutes in rats with normal lungs and found them equally effective.
In the development of the present invention however, it has been found that in the Application of the method employed by Hart et al to cats with normal lungs, there was severe progressive fall in functional residual capacity (FRC) which produced also a reduction in blood oxygen tension. Cats whose lungs have been rendered stiff by lavage with saline as a model of sick lung could not be successfully ventilated in this way nor even cats with normal lungs for a period more than a few minutes.